Herodotus, Histories 3.12
“The heads of the Persians are so weak that, if you were to toss a single pebble at one, you would make a hole in it. But the heads of the Egyptians are so strong that, using a stone, you could break one open only with a good deal of effort. They said that the reason for this (which I found easy to believe) was that the Egyptians, beginning straightaway in childhood, shave their heads and expose them to the sun, which hardens the bone. This is also the reason why Egyptians do not go bald: among the Egyptians one may observe the fewest balding men of any race in the world. This, then, is the reason that the Egyptians have such strong heads. The Persians have such weak heads for this reason: they are always wearing felt caps from the beginning of their lives.”
αἱ μὲν τῶν Περσέων κεφαλαί εἰσι ἀσθενέες οὕτω ὥστε, εἰ θέλεις ψήφῳ μούνῃ βαλεῖν, διατετρανέεις, αἱ δὲ τῶν Αἰγυπτίων οὕτω δή τι ἰσχυραί, μόγις ἂν λίθῳ παίσας διαρρήξειας. αἴτιον δὲ τούτου τόδε ἔλεγον, καὶ ἐμέ γ᾽ εὐπετέως ἔπειθον, ὅτι Αἰγύπτιοι μὲν αὐτίκα ἀπὸ παιδίων ἀρξάμενοι ξυρῶνται τὰς κεφαλὰς καὶ πρὸς τὸν ἥλιον παχύνεται τὸ ὀστέον. τὠυτὸ δὲ τοῦτο καὶ τοῦ μὴ φαλακροῦσθαι αἴτιον ἐστί: Αἰγυπτίων γὰρ ἄν τις ἐλαχίστους ἴδοιτο φαλακροὺς πάντων ἀνθρώπων. τούτοισι μὲν δὴ τοῦτο ἐστὶ αἴτιον ἰσχυρὰς φορέειν τὰς κεφαλάς, τοῖσι δὲ Πέρσῃσι ὅτι ἀσθενέας φορέουσι τὰς κεφαλὰς αἴτιον τόδε: σκιητροφέουσι ἐξ ἀρχῆς πίλους τιάρας φορέοντες.